Digital-to-analog converters



Dec. 25, v1962 H. E. ToMPKlNs 3,070,788

DIGITAL-To-ANALOG coNvERTERs Filed Dec. 1e. 195s F-i 1 l OUTPUT L A r g8 I VSf- 3? 37 37 v E52 39 L4o CLEAR '38 VT AND ZERO

S Zr` 26 |18 [61's elle M 5 A TTORNEY 3,970,788 Patented Dec. 25, i962 3,070,788 DIGlTAL-TO-ANALOG CONVERTERS Howard E. Tompkins, Swarthmore, Pa., assigner to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Filed Dec. 16, 1958, Ser. No. 780,866 3 Claims. (Cl. 340-347) This invention relates to signal conversion circuits and patricularly to circuits for digital-to-analog signal conversion.

The present invention is particularly adapted for use with a multiple output electron discharge counting device and provides a separate analog output signal at each output position of the device. These separate output signals are available at a single pair of output terminals having a constant output impedance and each output is independently adjustable without aecting any of the others.

Although there are many circuits known for performing digital-to-analog conversions, none of these includes the features of or provides the functions and advantages of the present invention.

The objects of the present invention are concerned with the provision of an improved signal conversion circuit and, particularly, the provision of an improved digital-to-analog conversion circuit.

Briey, a circuit embodying the invention utilizes a multiple output electron discharge device which includes, among other electrodes, a single cathode and a plurality of output electrodes. The output electrodes provide consecutive output current signals of constant amplitude. The output electrodes are coupled to an output circuit which comprises a network of parallel-connected tapped load resistors, each of which includes two end terminals and an adjustable tap. One end terminal of each resistor is connected to a rst common buss, and the other end of each resistor is connected to a second common buss. The variable tap on each resistor is connected to one of the output electrodes of the multiple output device. The busses, between which the load reisistors are connected, are coupled to output terminals between which the output voltages, generated by current ilow to each output electrode, appear.

In operation of the circuit, each time the discharge device performs a counting operation under the influence of an input digital signal, -a corresponding analog voltage is generated across the appropriate output load resistor and appears at the output terminals of the resistor network. The magnitude of the analog voltage at each position depends on the setting of the load resistor at that position. The analog voltage generated at each position may be varied by changing the resistor setting. The setting of each resistor and the analog voltage developed are substantially independent of all of the others, and each resistor may be set independently to provide a desired analog voltage without affecting the analog voltages generated at any of the other load resistors.

The invention is described in greater detail by 4reference to the drawing wherein:

FIG. 1 is a perspective view of a multiple output electron discharge device utilized in the present invention; and

FIG. 2 is a schematic representation of the device of FIG. l and a circuit in which it is operated according to the invention.

Referring to FIG. l, an electron discharge device which may be used in practising the present invention is of the type shown in U.S. Patent No. 2,721,955 to Fan et al. This type of tube includes, briey, an envelope 12 which contains a central longitudinally elongated cathode 14 and ten groups of electrodes spaced radially equidistantly from the cathode land surrounding the cathode. Each group of electrodes includes a generally U-shaped elongated spade electrode 16 and a generally L-shaped target electrode 1S positioned so that cach target occupies the l space between adjacent spade electrodes. Each spade electrode serves to form and hold an electron beam on its corresponding target electrode and a constant current is obtained from each target electrode. A generally rodlike switching electrode 20 is also included in each group of electrodes and is positioned between one edge of each target electrode and the adjacent spade electrode. The switching electrodes are known as switching grids. An open-ended cylindrical permanent magnet 22 is provided surrounding the tube envelope and coaxial therewith. The magnet provides an axial magnetic iield which is utilized in conjunction with electric elds within the tube to form and switch an electron beam from the cathode to each of the groups of electrodes. The direction in which the beam switches, that is clockwise or counterclockwise, is always the same and is determined by the orientation of the electric and magnetic elds.

Briefly, in operation of tube 10, electrons emitted by the cathode are retained at the cathode if each of the spades, targets and switching grids carries its normal operating electrical potential. When a spadey or switching grid experiences a suitable lowering of its potential, an electron beam is `formed and directed to the corresponding target electrode. The electron beam may be switched from one target electrode to the next by thus suitably altering the electrical potentials of a spade or switching grid. Under normal operating conditions, whenever electrode voltages are such that a beam might be supported at several positions, the beam will switch to the most loading position and lock in at this position.

in the circuit'o FIG. 2, the tube 1d is shown in schematic, linear form with the positions or groups of electrodes thereof numbered serially from 0 to 9. In the circuit, the cathode 14 is connected through a suitable resistor Z4 to ground. Each of the spade electrodes 16 is coupled through a spade load resistor 26 to a spade ibuss 28. The spade buss 28 is coupled through a common spa-de resistor 30 to a suit-able positive D.C. power supply Vs. A suitable zero-set circuit 32 for clearing an electron beam in the tube and resetting it at the 0 position is coupled to the spade buss and the spade' electrode at the 0 position. A typical zero-set circuit operates by irst reducing the potential of the spade buss to a level at which a beam cannot be maintained at any position and then holding the 0 spade at a lower voltage level than the other spades until a beam forms at the i0 position.

The switching grid electrodes Ztl may be connected in sever-al different ways. In one arrangement, shown in FIG. 2, the grids at the even-numbered positions are connected together in one set and the grids at the odd-nurnbered positions are connected together in another set. Each set of grids is then connected to oney of the outputs or" a suitable ip-tlop circuit 34. As rst one and then the other output of the flip-flop operates, the beam is moved from position to position in the tube.

According to the invention, the targets 18 are connected to an output circuit 36 as follows. The output circuit comprises a network made up of ten tapped load resistors 37 connected in parallel, one resistor for each output target electrode 18. One end of each resistor 37 is connected to a iirst buss 38, and the other end of each resistor is connected to a second buss 4d. The rst buss is provided with a rst output terminal 4Z, and the second buss is provided with a second output terminal 44 and is coupled to a suitable positive D.C. power supply VT. Output voltages are developed across each load resistor and between the two output terminals 42 and 44. An adjustable tap 46 on each resistor is connected to one of the target electrodes in the tube. Thus, each target is provided with an adjustable or variable load resistor.

in operation of the circuit of FIG. 2, an electron beam in the tube is switched from position to position by the operation of the flip-'hop circuit 34. As the beam ows to each position, output current ilows from the target electrode at the position through a load resistor 37. The output voltage which is developed across the variable resistor coupled to each output electrode depends on the setting Iof the adjustable tap 46. The setting of each resistor is independent of -all of the others, and the current ow through each and the output voltage developed across each one do not aect any of the others. Thus,

. the circuit of the invention provides a series of dilerent,

independent out-put pulses at the output terminals.

In using the circuit of FIG. 2 as a `digital-to-analog converter, the tube 1t)l is caused to execute a counting operation in response to a digital input signal. Thus, for example, a digital six input signal causes an electron beam in the tubeI to count to the 6 position. The resultant output voltage generated across Vthe variable load resistor 37 coupled to the target l18 at the 6 position and appearing at the out-put terminals 42 and `44 of the resistor network is the equivalent analog output voltage of the digital siX. Subsequently, if the load resistor at the 6 position is changed, the resultant analog voltage for a digital six count will have a different value. Thus, the analog voltage generated across each load resistor may be varied. However, any such variation is achieved without affecting any of the other loads and without affecting the characteristic output impedance of the network.

What is claimed is:

l. A signal generating circuit including a multiple output electron discharge device having a cathode for genererating an electron beam and electrode means for switching an electron beam sequentially through a plurality of positions in said device,

a plurality of output electrodes in said discharge device each of which is adapted to receive an electron beam from said cathode and provide an output signal when an electron beam flows thereto,

a plurality of impedances connected in parallel in a network,

each impedance having an adjustable tap with each tap being coupled to one output electrode so that when an electron beam ows to an output electrode, output current therefrom ows through a tap and ya portion of an impedance,

a pair of output terminals provided on said network and coupled each to one end of each of said impedances, the output current `from each output electrode providing an output potential at said output terminals,

the amplitude of the output potential provided by each `out-put electrode depending on the setting of the tap on the impedance coupled thereto,

the impedance looking into said terminals being substantially constant Whatever the settings of said adjustable taps.

2. The circuit dened in claim `1 wherein said impedances comprise adjustable resistors.

3. A signal generating circuit including a multiple output electron tube having a central cathode and a plurality of groups of electrodes surrounding said cathode,

each group of electrodes comprising a position to which an electron beam may rlow from said cathode,

each of said groups of electrodes including a target output electrode from which an output signal flows and a spade electrode which forms and holds an electron beam on its target electrode,

a plurality of load resistors connected in parallel to form a network,

a pair of output terminals coupled to said network with each terminal coupled to one end of each resistor, each resistor having an adjustable tap coupled to a separate one of said target electrodes so that when an electron beam flows to a target electrode, output current flows through a tap `and a portion of one `of said resistors and provides an output potential at said output terminals,

the amplitude of the output potential provided by each output electrode depending on the setting of the tap on the resistor coupled thereto, the impedance looking into said terminals being substantially constant regardless of the settings of said adjustable taps.

References Cited in the le of this patent UNITED STATES PATENTS 2,445,682 Macgeorge July 20, 1948 2,661,899 Chromy et al Dec. 8, 1953 2,784,396 Kaiser Mar. 5, 1957 2,853,694 Metzger Sept. 23, 1958 2,869,115 Doleman Jan. 13, 1959 2,881,418 Stephens Apr. 7, 1959 

